When exercise shifts from a healthy habit to an unhealthy addiction

— Even if Abby Heugel wanted to stop, she couldn't.

She had to sweat. She had to feel her heart race and her muscles stretch, contract and burn. She had to be in control. She had to exercise.

Heugel, 35, has a history of depression, obsessive-compulsive disorder and exercise addiction, which has left her underweight for a decade, she said. Although she sees a therapist, she said she struggles with her addiction every day.

"I physically feel like I will jump out of my skin if I don't move every couple of hours. Mentally, it's torture," said Heugel, a writer based in Grand Rapids, Michigan.

Heugel has often shared stories about her depression and OCD in her work, in a snarky, tongue-in-cheek way, with dry humor -- but she says she rarely opened up about her exercise addiction.

Many people don't seem to understand how exercising can evolve from a healthy habit to a potentially harmful addiction, she said, and she wants to change that.

"I would define it as part of my anxiety and OCD. I don't over-exercise because I think I'm fat and need to lose weight. In fact, it's the complete opposite. ... If I could have 40 pounds put on my frame tomorrow, I would do it in a heartbeat, which is why so many people are confused," Heugel said.

"Why not eat more? I do eat more than a normal person, but it's all very controlled and obsessive as well and not enough to sustain my overactivity. Why not just rest? Because like alcohol or drugs, it's an addiction," she said. "It's what I do when I'm anxious; it's part of my routines. It's a compulsion."

Exercise addiction is not included in the Diagnostic and Statistical Manual of Mental Disorders, the authoritative guide to defining, classifying and diagnosing mental health disorders. The only behavior-associated addiction in the DSM is gambling.

However, a paper that published last month in the British Medical Journal encourages health-care professionals to recognize and understand the risks of exercise addiction.

Symptoms of exercise addiction appear in about 0.3% to 0.5% of the general population worldwide, said Heather Hausenblas, a professor of kinesiology at Jacksonville University in Florida and lead author of the paper.

"It's a small percentage, but ... if you're taking a look at the whole country, it's hundreds of thousands of individuals who have this," Hausenblas said.

"We tend to -- rightfully so -- think of exercise as a really positive thing we need to be doing, and most of us don't exercise enough and aren't getting a hold of the health-related benefits of exercise," she said. "But like with any behavior, we can take it to an extreme."

When exercise is taken to an extreme, Hausenblas said, it can manifest as a secondary addiction, in which it's secondary to an eating disorder and an individual is exercising only to control or maintain their weight. Or exercise addiction may manifest as a primary addiction, in which there is no underlying pathology.